This invention relates to digital radiography, and in particular to the imaging of a long human body part, such as the spine or the legs, using direct digital x-ray detector system.
When a long segment of the human body is imaged using a conventional screen-film technique, special cassettes and films of extended length are used, such as 30xc3x9790 cm and 35xc3x97105 cm. As shown in FIG. 1, both the x-ray tube 101 and the cassette 103 are kept stationary during the exam, an image of the patient is acquired in a single exposure. As medical institutions are migrating from analog screen-film systems to digital modalities, such as flat-panel detector based digital radiography (DR) systems, this type of exam imposes a significant challenge. This is because the sizes of digital detectors are limited by cost and technology. The largest flat panel DR plates available today are limited to 43xc3x9743 cm. A detector of this size is capable of imaging only a portion of the body part at a time and thus is inadequate for performing imaging exams of longer length body parts such as the full spine or full leg. To address this problem, multiple exposures at varying patient positions must be acquired with the assumption that the patient can keep still during the exam. The individual images are then stitched together to reconstruct a larger composite image (see: U.S. Pat. Nos. 5,123,056 and 4,613,983). FIG. 2A shows the most commonly used approach with the DR systems. First, the patient 200 is exposed at a first position defined by the x-ray tube position 201 and the detector position 203. The collimator of the x-ray tube is adjusted such that the x-rays 202 can barely cover the detector while protecting the patient from unnecessary radiation in the non-imaging related regions. Second, after both the x-ray tube and the detector are translated parallel along axis 210 and 211 to a second position, as indicated by 206 and 208, a second exposure of the patient is conducted. There can be a slight overlap between the consecutive detector coverage in order to facilitate image stitching. This process can be continued until the full length of the body part to be examined is imaged. However, this stereovision-like image acquisition geometry has two major drawbacks: (1) mechanical complexity because both the detector and the x-ray tube need to be translated, and (2) inherent geometric distortion that makes precise image stitching almost impossible. As shown in FIG. 2B, if an object AB is situated in both the exposure coverage of x-ray tube position 201 and 206, it will be imaged at the corresponding detector position as AB0 and AB1, respectively, due to the divergence of the x-rays. With AB0 pointing downward but ABa pointing upward, the same object has apparently created two different images. Evidently, AB0 and AB1 can not be registered together in the stitched image. Therefore, theoretically the images acquired at different tube positions can never be seamlessly and precisely stitched. This severity of this problem becomes worse if the body part get thicker. There is a need to develop an imaging method with DR that not only is simple in design but also can provide distortion-free images for stitching.
According to the present invention, there is provided a solution to the problems and a fulfillment of these needs.
According to a feature of the present invention, there is provided a method for acquiring a radiographic image of an elongated object, comprising: positioning an elongated object between a source of x-rays and a digital image capture device having an imaging dimension which is less than a like dimension of said elongated object; wherein said image source is stationary and said source projects an x-ray pattern which encompasses said elongated object; moving said digital image capture device in a direction parallel to said known imaging dimension to sequential contiguous stationary positions to acquire a sequence of radiographic images of said elongated object; and moving an assembly of x-ray opaque material which is positioned between said source of x-rays and said elongated object, and which has an opening for allowing passage of x-rays, such that said opening is synchronized with the movement of said digital image capture device to facilitate acquisition of said sequence of radiographic images at said stationary positions.
The invention has the following advantages.
1. The new method eliminates the stereovision-like geometric distortion caused by the current imaging method with DR systems, and allows precise registration of the partial images to reconstruct a larger composite image.